1. Field of the Invention
The present invention relates to an electrostatic or electrophotographic image forming apparatus.
2. Description of the Related Art
An intermediate transfer type image forming apparatus shown in FIG. 8 is an example of a color image forming apparatus serving as a photocopier or a printer. The illustrated image forming apparatus sequentially superposes toner images formed in a plurality of image forming stations 10 that form toner images of respective colors, on an intermediate transfer member (intermediate transfer belt 31) and then transfers the toner images together onto a transfer material P.
As shown in FIG. 9, the intermediate transfer belt 31 is moved in the direction of an arrow by the rotation of an intermediate transfer belt drive roller 32. The drive input into the drive roller 32 is performed from a body drive or a stepping motor via a coupling 67 or a rocking gear. Registration patterns 65 and 66 formed on the intermediate transfer belt 31 by the image forming stations 10 are detected by a registration sensor unit 60. On the basis of the detection results, image adjustment is performed. The registration sensor unit 60 is positioned by positioning units 64 provided in a frame of an intermediate transfer unit 30, so as to be perpendicular to the height direction of the intermediate transfer belt 31 and the moving direction of images in a first transfer plane A.
The registration sensor unit 60 includes sensors 61 (registration sensors) disposed at predetermined positions in front of and behind the images. Each registration sensor 61 includes a light emitter 62 and a light receiver 63. Light emitted from the light emitter 62 is reflected by the intermediate transfer belt 31 and is then received by the light receiver 63. The parts that bear toner (registration patterns 65 and 66) reflect a smaller amount of light to the light receiver 63. The parts that do not bear toner reflect a larger amount of light to the light receiver 63. The registration patterns 65 and 66 are detected from the amount of reflected light that the light receiver 63 receives.
The amount of deviation of the start position of each subscanning and the inclination are detected by using the registration pattern 65. The amount of deviation of the start position of the main scanning and the overall magnification ratio are detected by using the registration pattern 66. On the basis of these data, the deviation of the timing of starting the image formation in the main scanning and the subscanning, the change in magnification ratio, the inclination of the image, and so forth are adjusted by image correction (hereinafter referred to as “registration correction” or “automatic registration”). See, for example, Japanese Patent Laid-Open No. 2004-029525.
This image adjustment (hereinafter referred to as “automatic registration mode”) is performed, for example, when the image forming apparatus is turned on, at the restart after the clearance of a paper jam, and after a lapse of a predetermined time period from the power-on. Other than this automatic registration, for example, some image forming apparatuses form toner images for detection for performing density adjustment and execute density adjustment on the basis of these toner images for detection.
However, in the case of a conventional image forming apparatus, if anyone or anything bumps against the apparatus and causes a vibration during or after the automatic registration mode or the density adjustment, an image formation failure can occur. For example, if the image forming apparatus is substantially vibrated, an image cannot be formed at the target position, and density fluctuation occurs, and thereby an image formation failure occurs. In particular, if a vibration occurs during a full color image formation, in which images are superposed, and thereby the image forming position is deviated, an image formation failure such as color misregistration occurs.